When you visit the main page of the Town Square (http://www.ghostvill...in/yabb/YaBB.pl) you will see a little folder icon on the left-hand side of the page next to each board. If the folder has a blue dot in it, that means there are messages in there you haven't read. If the dot is silver that means there are no new messages in that board.

Once you click into a specific board you will see a small graphic that says "new" next to any messages that have been updated since you read them last.